The present invention relates to polyethylene naphthalate film for high-density magnetic recording media of floppy disk. The present invention particularly relates to a polyethylene naphthalate film for high-density magnetic recording media of floppy disk which has little anisotropy, high mechanical strength and good dimensional stability, and which scarcely causes a drop out and has good productivity. More particularly, the present invention relates to a polyethylene naphthalate film which has a tensile stress at 5% elongation (hereinunder referred to as "F-5 value") in the range of 13 kg/mm.sup.2 to 21 kg/mm.sup.2 in any direction on the film; a Young's modulus in the range of 500 kg/cmm.sup.2 to 800 kg/mm.sup.2 in any direction on the film; a heat shrinkage of less than 2.5% in any direction; a center line average roughness Ra of not more than 0.012 .mu.m and not more than 10 coarse protuberances of not lower than 1.08 .mu.m per 25 cm.sup.2, wherein each of the deviations of the F-5 values, Young's moduli and heat shrinkages is not more than 20%.
A magnetic recording disk is obtained by forming a magnetic layer on the surface of a base film by applying magnetic particles together with a binder, or coating the surface of the base film with magnetic metal by vacuum deposition, sputtering or plating. Demand for high-density recording on such magnetic recording media of floppy disk has recently been increasing. With such an increasing demand for high-density recording of floppy disk, much importance has been taken on the improvements of the heat resistance, physical properties (tensile stress at 5% elongation, Young's modulus, heat shrinkage, etc.), and the isotropy for magnetic recording media. If a magnetic recording medium is made of a base film insufficient in isotropy, namely, a base film having much anisotropy, it twists, causes thermal strain, or is inferior in toughness, so that it is unsuitable for high-density recording and reproduction. Furthermore, with a tendency to high-density recording, a film is required to have evenness. However, mere reduction in the surface roughness does not bring about a highly densified magnetic recording material. That is, it is necessary to reduce the number of the coarse protuberances as well as the surface roughness in order to prevent drop outs.
Polyethylene terephthalate films have conventionally been used as a base film for floppy disk. Improvements of these films have also been attempted to meet the demand for high-density magnetic recording media. Polyethylene terephthalate, however, does not have sufficient heat resistance, and it is difficult both to reduce the anisotropy of the film and to increase its mechanical strength. Thus, a film which possesses all the properties required for a high-density magnetic recording medium of floppy disk has been strongly demanded.
On the other hand, since polyethylene naphthalate which has a naphthalene ring in a molecular chain has a rigid molecular structure and exhibits a high crystallizability, a film formed of this material is expected to be excellent in heat resistance, mechanical strength and dimensional stability, and a magnetic recording medium adopting a polyethylene-2,6-naphthalate film as a base film and its manufacturing method have already been proposed (Japanese Patent Publication No. 48-29541 (1973), Japanese Patent Application Laying-Open (KOKAI) No. 50-45877 (1975) and Japanese Patent Publication No. 56-19012 (1981)). However, since these polyethylene-2,6-naphthalate films disclosed in the prior art are insufficient in isotropy of the mechanical properties, they are unsatisfactory to be used as a base film for a high-density magnetic recording medium of floppy disk.
As a result of various researches of the present inventor to provide a film suitable as a base film for a high-density magnetic recording medium of floppy disk it has been found that a film obtained by providing a substantially non-oriented polyethylene naphthalate film with specific properties by means of sequential biaxial orientation, in other words, a polyethylene naphthalate film which has an F-5 value in the range of 13 kg/mm.sup.2 to 21 kg/mm.sup.2 in any direction on the film; a Young's modulus in the range of 500 kg/mm.sup.2 to 800 kg/mm.sup.2 in any direction on the film; a heat shrinkage of less than 2.5% in any direction; a center line average roughness Ra of not more than 0.012 .mu.m and not more than 10 coarse protuberances of not lower than 1.08 .mu.m per 25 cm.sup.2, wherein each of the deviations of the F-5 values, Young's moduli and heat shrinkages is not more than 20% possesses all the properties required for a base film for a high-density magnetic recording medium of floppy disk. The present invention has been accomplished on the basis of this finding.